It's not possible to have a proper canonical question about medicine safety that covers all animal species properly. Beyond the obvious: only on advice from a vet, there are many variations in size, weight, biochemistry, etc. that make any meaningful response species specific. I've reverted some of the changes as a result, the responses here are dog oriented.
– John CavanApr 27 '14 at 15:56

4 Answers
4

People may be tempted to try and save money, by bypassing a trip to the Vet or paying for expensive Veterinary medications and products. Unfortunately the cost of looking after our pets can be high, but to try and help our pets with well meaning shortcuts, by using human products and medications is not a viable solution.

Animals metabolise drugs differently from human beings and differently between species. What may be effective in a human being, may be harmful or potentially fatal for a dog.

Dog poison No. 1: Prescription medications for people. Drugs that
might be beneficial or even lifesaving for people can have the
opposite effect in pets. And it doesn’t always take a large dose to do
major damage.

Some of the most common and harmful medications that poison dogs
include:

Medications that can be toxic at certain doses include: Aspirin, Paracetamol and Ibuprofen. Other medications that should not be given to your dog as they can be potentially lethal, even in small doses, include:

Antidepressants – can cause vomiting and lethargy with certain types leading to serotonin syndrome.ADHD medications – act as a stimulant and dangerously elevate heart rate, blood pressure and body temperature.Anti-cancer drugs.Anti-diabetics – cause a major drop in blood sugar levels causing disorientation, lack of coordination and seizures.Cold medicines – acts as a stimulant causing elevated heart rates, blood pressure, body temperature and seizures.Vitamin D derivatives – cause life-threatening spikes in blood calcium levels in pets that can lead to kidney failure.Diet pills.Muscle relaxants – can impair the central nervous system and lead to death. (2)

It's important never to assume:

An important note to make is, although many medications may be the same that is used for a pet and a human, it is important not to dose a pet based on the dosage given to a person. It is not just a weight by weight correlation, different species may require a varying dose per weight, as they have different metabolisms.

The only exception:

On the written advice of a Veterinarian:
For instance, you take your dog to the Veterinarian and the Vet gives you written instructions on the use of any over the counter medications with explicit dosage guides.

References:

WebMD (1)

Department of Environment and Primary Industries (DEPI)
Victorian State Government Australia (2)

Our vet has suggested we use specific human over-the-counter medicine on our dog in specific cases, however she has also said we need to pay careful attentions to dosages.

The most common specific case is Benadryl for allergy relief, and I know others who have been told the same. Children's Benadryl is preferred but they don't make that in pill form anymore, so we either have to get him to take a liquid syrup or cut an adult Benadryl in half.

I can't speak for Children's cough syrup though as I don't have any direct experience, but based on some outside references, it is ok for a minor case for very short time periods. It should be avoided however because cough syrups suppress the cough but do not treat the condition causing the cough, and hence could delay treatment for a more serious problem.

One of my most common human OTC medications, pain relievers, are a big no-no though. A dogs liver cannot process pain relievers the same way human's can, so it can kill them or at least damage their stomach or liver, especially Advil/Mortin (Ibuprofen) and Tylenol (Acetaminophen). Aspirin, specifically buffered Aspirin, is the safest of the 3 and it is still not recommended.

It should be noted that the name brand Benadryl is often formulated differently than the generic/store brands. There are some generics that use ingredients that can be bad for your animal.
– user9Oct 17 '13 at 21:29

I would say any human medicine is for exactly that... for humans only. Any usage of human medicine on any other species probably hasn't been widely tested. I doubt many humans would be willing to take medicine/drugs that were originally created for a dog/monkey/giraffe/fish without first consulting a doctor.

Any direction of use of human medicine should be given from someone with direct knowledge of the situation and has the training and/or background to make educated decisions (i.e. veterinarian)

Since drugs and compounds may behave differently in animals than in humans, veterinarians should ask compounding pharmacists if they have specialized training or credentials in veterinary compounding, and veterinarians should specifically seek the compounding services of pharmacists who have specialized knowledge of veterinary compounding.

Yes, but remember the dosage that is intended to give for Humans and pets is not the same. so anyone should be very careful about how much dosage you actually give to it. For example as like people above mentioned Tylenol is one such medicine used for Dogs as a pain killer, but the dark side is, it is used to euthanize dogs, due to its toxic effect when used over dose.